Liquid crystal display device and manufacturing method thereof

ABSTRACT

The present invention provides a liquid crystal display device in which even in the case of using a photo-alignment technique, excellent afterimage characteristics can be stably obtained. Provided is a liquid crystal display device including a TFT substrate having an alignment film, an opposed substrate which is arranged to face the TFT substrate and on which an alignment film is formed, and a liquid crystal layer sandwiched between the alignment films, wherein the alignment films are materials that can provide a liquid crystal alignment restraining force by irradiating polarized light, and the ratio of oxygen atoms on the surface of the alignment film is higher than that in the alignment film.

CLAIM OF PRIORITY

The present application claims priority from Japanese patent applicationJP 2013-111016 filed on May 27, 2013 the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a liquid crystal display device and amanufacturing method thereof.

2. Description of the Related Art

A liquid crystal display device has been widely used in various fieldsbecause. of the features such as high display quality, a smallthickness, a light weight, and low power consumption, and thus has beenwidely used for monitors for mobile devices such as those for mobilephones and digital still cameras, monitors for desktop personalcomputers, monitors for printing and designing, monitors for medicaluse, and liquid crystal televisions. Along with the expansion inapplication, higher image quality and higher quality have been requiredfor the liquid crystal display device. In particular, high brightness byhigh transmittance and low power consumption have been stronglyrequired. Further, along with the popularization of the liquid crystaldisplay device, there has been a strong need for low costs.

In general, a liquid crystal display device displays an image usingchanges of the optical characteristics of a liquid crystal layer causedby changing the alignment direction of liquid crystal molecules whileapplying an electric field to the liquid crystal molecules of the liquidcrystal layer sandwiched between a pair of substrates. The alignmentdirection of the liquid crystal molecules when no electric field isapplied is regulated by an alignment film obtained by performing arubbing process on the surface of a polyimide thin film. A conventionalactive driving-method liquid crystal display device having a switchingelement such as a thin-film transistor (TFT) for each pixel displays animage in such a manner that an electrode is provided at each of a pairof substrates sandwiching a liquid crystal layer to be set as aso-called vertical electric field in which the direction of an electricfield applied to the liquid crystal layer becomes substantially verticalto the surface of the substrate, and the optical rotation of liquidcrystal molecules configuring the liquid crystal layer are used. Astypical vertical electric-field liquid crystal display devices, a TN(Twisted Nematic) method and a VA (Vertical Alignment) method are known.

In the TN or VA liquid crystal display device, the viewing angle isdisadvantageously narrow. Accordingly, an IPS (In-Plane Switching)method and an FFS (Fringe-Field Switching) method are known as displaymethods to achieve a wide viewing angle. The IPS method and the FFSmethod are so-called horizontal electric-field display methods in whicha comb-like electrode is formed at one of a pair of substrates, and anelectric field generated has components substantially parallel to thesurface of the substrate. Liquid crystal molecules configuring a liquidcrystal layer are allowed to be rotated in a plane substantiallyparallel the substrate, and the birefringence of the liquid crystallayer is used to display an image. Due to the in-plane switching of theliquid crystal molecules, the viewing angle is advantageously wider andthe load capacity is lower as compared to the conventional TN method.Thus, these methods have been highly expected as new liquid crystaldisplay devices in place of the TN method, and have been rapidlyprogressed in these days.

A liquid crystal display element controls the alignment state of theliquid crystal molecules in the liquid crystal layer on the basis of thepresence or absence of electric fields. Specifically, upper and lowerpolarizing plates provided outside the liquid crystal layer are placedto be completely orthogonal to each other, and a phase difference isallowed to be generated using the alignment state of intermediate liquidcrystal molecules to form bright and dark states. The alignment statewhen no electric field is applied to liquid crystal can be controlled insuch a manner that a polymer thin film called “alignment film” is formedon the surface of a substrate, and the liquid crystal molecules arealigned in the polymer array direction using an intermolecular mutualeffect by the van der Waals force of polymer chains and liquid crystalmolecules at the interface. The effect is called as providing analignment restraining force or a liquid crystal alignment capability, oran alignment process.

Polyamide is used for the alignment film of a liquid crystal display inmany cases. As a formation method, polyamide acid as the precursor ofpolyimide is dissolved in various solvents to be applied onto asubstrate by spin coating or printing. Then, the substrate is heated ata temperature of 200° C. or higher to remove the solvents, and polyamideacid is put to imidization ring-closing reaction to polyimide. The filmthickness at this time is as thin as about 100 nm. The surface of thepolyimide thin film is rubbed in a certain direction using a rubbingcloth, so that the polyimide polymer chains on the surface are alignedin the direction to realize a state in which the anisotropy of polymeron the surface is high. However, there are problems such as generationof static electricity and foreign substances caused by rubbing andunevenness of rubbing due to irregularities of the surface of thesubstrate. Thus, a photo-alignment method that is not required to becontacted with the rubbing cloth and that controls the molecularalignment using polarized light is beginning to be employed.

The photo-alignment methods of the liquid crystal alignment film includea photoisomerization type in which polarized ultraviolet rays such asazo color are irradiated to change the geometric arrangement inmolecules, and an optical dimeric type in which molecular frameworkssuch as cinnamic acid, coumalin, and chalcone are chemically bonded bypolarized ultraviolet rays. However, a photodegradative type in whichpolarized ultraviolet rays are irradiated onto polymer to cut anddecompose only the polymer chains aligned in the direction, and thepolymer chains in the direction orthogonal to the polarized directionare left is suitable for photo-alignment of polyimide that is reliableand proven as the liquid crystal alignment film.

The principle of such a photo-alignment method is disclosed in, forexample, “Nematic Homogeneous Alignment by Photo Depolymerization ofPolyimide, by Masaki HASEGAWA and Yoichi TAIRA, proceedings of 20thdiscussion on liquid crystal, pp. 232 to 233, 1994”. The method wasstudied using various liquid crystal display methods, among which theIPS method is disclosed in Japanese Patent Application Laid-Open No.2004-206091 as a liquid crystal display device that reduces generationof display fault due to variation of the initial alignment direction,has stable alignment of liquid crystal, is suitable for mass production,and has high-quality images in which the contrast ratio is increased.Japanese Patent Application Laid-Open No. 2004-206091 shows thatalignment control capability is provided by an alignment process inwhich at least one of secondary processes of heating, irradiation ofinfrared rays, irradiation of far-infrared rays, irradiation of electronbeams, and irradiation of radioactive rays is performed for polyamicacid or polyimide composed of cyclobutanetetracarboxylic dianhydrideand/or the derivative thereof and aromatic diamine.

Then, Japanese Patent Application Laid-Open No. 2004-206091 shows thatespecially, at least one of processes of heating, irradiation ofinfrared rays, irradiation of far-infrared rays, irradiation of electronbeams, and irradiation of radioactive rays is performed together with apolarization irradiation process, so that the alignment process can befurther effectively performed. Further, Japanese Patent ApplicationLaid-Open No. 2004-206091 shows that an imidization baking process of analignment control film is performed together with a polarizationirradiation process, so that the alignment process can be furthereffectively performed. In particular, in the case where at least one ofprocesses of heating, irradiation of infrared rays, irradiation offar-infrared rays, irradiation of electron beams, and irradiation ofradioactive rays is performed for the liquid crystal alignment film inaddition to the polarization irradiation process, the temperature of thealignment control film is desirably in a range of 100° C. to 400° C.More desirably, the temperature is in a range of 150° C. to 300° C.Japanese Patent Application Laid-Open No. 2004-206091 shows that theprocesses of heating, irradiation of infrared rays, and irradiation offar-infrared rays are effective because the processes can be performedtogether with the imidization baking process of the alignment controlfilm.

However, the history of development of the liquid crystal display deviceusing the photo-alignment film is shorter as compared to that using therubbing alignment film, and there are not sufficient findings in termsof long-time display quality over several years as a practical liquidcrystal display device. Specifically, the fact is that a relationbetween image fault that does not become obvious in the initial stage ofmanufacturing and a problem unique to the photo-alignment film has beenhardly reported.

SUMMARY OF THE INVENTION

The inventors concluded that a photo-alignment technique would becomeimportant in order to realize high-quality and high-definition liquidcrystal display devices in the future, and studied, in detail, issues ofapplying the photo-alignment technique to the liquid crystal displaydevices. As a result, it was found that the photo-alignment technique iseffective for the problems of generation of static electricity andforeign substances and unevenness due to irregularities of the surfaceof the substrate as compared to the rubbing technique. However, it wasfound that the photo-alignment technique is disadvantageous when adaptedto products in the future in terms of afterimage characteristics.

An object of the present invention is to provide a liquid crystaldisplay device and a manufacturing method thereof in which even in thecase of using a photo-alignment technique, excellent after imagecharacteristics can be stably obtained.

The following is a summary of the representative embodiment of theinvention disclosed in the application.

The present invention provides a liquid crystal display deviceincluding: a TFT substrate which includes pixel electrodes and TFTs andon which an alignment film is formed on a pixel; and an opposedsubstrate which is arranged to face the TFT substrate and on which analignment film is formed on the uppermost surface on the side of the TFTsubstrate, wherein liquid crystal is sandwiched between the alignmentfilm of the TFT substrate and that of the opposed substrate, thealignment film is a hydrophobic material that can provide a liquidcrystal alignment restraining force by irradiating polarized light, alayer in which the ratio of elements configuring the alignment film ischanged towards the film thickness direction is provided on the surfaceof the alignment and the ratio of oxygen atoms on the surface of thealignment film is higher than that inside the alignment film in a statewhere hydrophobicity is maintained.

Further, in the liquid crystal display device, the ratio of oxygenconfiguring the alignment film of the layer in which the ratio ofelements configuring the alignment film is changed towards the filmthickness direction is gradually decreased from the surface of thealignment film towards the inside of the alignment film.

Further, in the liquid crystal display device, the ratio of oxygen at aposition with the highest oxygen concentration in the layer in which theratio of elements is changed towards the film thickness direction higherthan that at a position with the lowest oxygen concentration by 25% orhigher.

Further, in the liquid crystal display device, the thickness of thelayer in which the ratio of elements is changed towards the filmthickness direction is smaller than the entire thickness of thealignment film by 50% or smaller.

Further, in the liquid crystal display device, the root-mean-square ofthe size of irregularities on the surface of the alignment film is 1 nmor smaller.

Further, in the liquid crystal display device, the alignment film is aphotodegradation-type photo-alignment film.

Further, in the liquid crystal display device, the alignment film is aphotodegradation-type photo-alignment film, including polyimide of(Chemical formula 1) in which the inside of the parentheses [ ]represents a chemical structure of a repeating unit, the index nrepresents a number of a repeating unit, N represents nitrogen atoms, Orepresents oxygen atoms, A represents a quadrivalent organic groupincluding a cyclobutane ring, and D represents a divalent organic group.

Further, in the liquid crystal display device, the alignment film has astructure in which two kinds of layers are laminated, and is of atwo-layer structure including a photo-alignment upper layer capable ofphoto-alignment and a low-resistive lower layer that is smaller inresistivity than the photo-alignment upper layer.

Further, in the liquid crystal display device, the liquid crystaldisplay device is an IPS liquid crystal display device.

The layer in which the ratio of elements configuring the alignment filmis changed towards the film thickness direction means a layer in whichwhen analyzing the elemental composition of the alignment film, thecomposition in the plane of the film is constant, but when analyzing thecomposition in the plane in the film thickness direction, the elementalcomposition is changed. The present invention is characteristic in usingthe alignment film including such a layer. The state in which the ratioof oxygen configuring the alignment film is gradually decreased from thesurface of the alignment film towards the inside of the alignment filmmeans a state in which when analyzing the composition in the plane inthe film thickness direction, the composition of oxygen is decreasedwithout increasing in the middle as the position from the surface of thefilm becomes deeper.

Further, polyimide in this case is a highly-polymerized compoundrepresented by (Chemical formula 1) in which the inside of theparentheses [ ] represents a chemical structure of a repeating unit, theindex n represents a number of a repeating unit, N represents nitrogenatoms, O represents oxygen atoms, A represents a quadrivalent organicgroup, and O represents a divalent organic group. As an example, thestructure of A includes an aromatic ring system compound such as aphenylene ring, a naphthalene ring, or an anthracene ring, an aliphaticring system compound such as cyclobutane, cyclopentane, or cyclohexane,or a compound obtained by bonding a substituent to these compounds.Further, as an example, the structure of D includes an aromatic ringsystem compound such as phenylene, biphenylene, oxyphenylene,biphenylene amine, naphthalene, or anthracene, an aliphatic ring systemcompound such as cyclohexene or bicyclohexane, or a compound obtained bybonding a substituent to these compounds.

Polyimide is applied onto an underlayer held in a substrate in the stateof the precursor of polyimide.

Further, the precursor of polyimide in this case is polyamide acid or apolyamide acid ester highly-polymerized compound represented by(Chemical formula 2). In this case, H represents hydrogen atoms, R1 andR2 represent hydrogen or alkyl chains of —C_(m)H_(2m+1), and m is 1 or2.

Such an alignment film can be formed by a general formation method of apolyimide alignment film as follows. For example, an underlayer iscleaned using various surface processing methods such as a UV/ozonemethod, an excimer UV method, and an oxygen plasma method. Then, theprecursor of the alignment film is applied using various printingmethods such as screen printing, flexographic printing, and ink etprinting, and a leveling process is performed to have a uniform filmthickness under a predetermined condition. Then, for example, the filmis heated at a temperature of 180° C. or higher, and polyamide as theprecursor is put to imidization reaction to polyimide, so that a thinfilm can be formed. Further, polarized ultraviolet rays are irradiatedand a proper post-process is performed using desired means, so that analignment restraining force can be generated on the surface of thepolyimide alignment film. The upper and lower substrates with thealignment films thus formed are pasted together while keeping a certaingap. Then, a portion in the gap is filled with liquid crystal, and anend portion of the substrate is sealed, so that a liquid crystal panelis completed. Then, optical films such as a polarizing plate and aretardation film are pasted to the panel, and a driving circuit, abacklight, and the like are attached, so that the liquid crystal displaydevice can be obtained.

Further, the present invention provides a manufacturing method of aliquid crystal display device including: a TFT substrate which includespixel electrodes and TFTs and on which an alignment film is formed on apixel; and an opposed substrate which is arranged to face the TFTsubstrate and on which an alignment film is formed on the uppermostsurface on the side of the TFT substrate, wherein liquid crystal issandwiched between the alignment film of the TFT substrate and that ofthe opposed substrate, the method including the steps of: preparing theTFT substrate including the pixel electrodes and the TFTs; forming thehydrophobic alignment film on the TFT substrate; and increasing theratio of oxygen atoms on the surface of the alignment film in a statewhere hydrophobicity is maintained while generating an alignmentrestraining force on the alignment film by irradiating ultraviolet raysonto the alignment film and by performing an oxidation process for thealignment film.

Further, the present invention provides a manufacturing method of aliquid crystal display device including: a TFT substrate which includespixel electrodes and TFTs and on which an alignment film is formed on apixel; and an opposed substrate which is arranged to face the TFTsubstrate and on which an alignment film is formed on the uppermostsurface on the side of the TFT substrate, wherein liquid crystal issandwiched between the alignment film of the TFT substrate and that ofthe opposed substrate, the method including the steps of: preparing theopposed substrate; forming the hydrophobic alignment film on the opposedsubstrate; and increasing the ratio of oxygen atoms on the surface ofthe alignment film in a state where hydrophobicity is maintained whilegenerating an alignment restraining force on the alignment film byirradiating ultraviolet rays onto the alignment film and by performingan oxidation process for the alignment film.

The following is a summary of effects obtained by the representativeembodiment of the invention disclosed in the application.

Even in the case of using a photo-alignment technique, the ratio ofoxygen atoms on the surface of an alignment film is increased whilehydrophobicity on the surface of the alignment film is maintained, sothat it is possible to obtain a liquid crystal display device and amanufacturing method thereof in which excellent afterimagecharacteristics can be stably obtained while preventing absorption ofcontaminated objects on the alignment surface and accumulation ofresidual electric charge without deteriorating alignmentcharacteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a structure of an alignment film in aliquid crystal display device according to an embodiment of the presentinvention;

FIGS. 2A to 2C are schematic views of changes in the ratio of oxygenatoms from the surface of the alignment film to the depth direction inthe liquid crystal display device according to the embodiment of thepresent invention, in which FIG. 2A shows a case in which the ratio ofoxygen atoms is decreased from the surface to the inside, and then isincreased more than the ratio of oxygen atoms contained in an innerlayer, FIG. 2B shows a case in which the ratio of oxygen atoms isdecreased from the surface to the inside, and then becomes substantiallythe same as the ratio of oxygen atoms contained in the inner layer, andFIG. 2C shows a case in which the ratio of oxygen atoms is decreasedfrom the surface to the inside, and then is decreased less than theratio of oxygen atoms contained in the inner layer;

FIG. 3A is a schematic block diagram for showing an example of anoutline configuration of the liquid crystal display device according tothe embodiment of the present invention;

FIG. 3B is a schematic circuit diagram for showing an example of acircuit configuration of one pixel of a liquid crystal display panelshown in FIG. 3A;

FIG. 3C is a schematic plan view for showing an example of an outlineconfiguration of the liquid crystal display panel shown in FIG. 3A;

FIG. 3D is a schematic cross-sectional view for showing an example of across-sectional configuration taken along the line A-A′ shown in FIG.3C;

FIG. 4 is a schematic cross-sectional view for showing an example of anoutline configuration of main parts (IPS liquid crystal display panel)in the liquid crystal display device according to the embodiment of thepresent invention;

FIG. 5 is a schematic cross-sectional view for showing an example of anoutline configuration of main parts (FFS liquid crystal display panel)in the liquid crystal display device according to the embodiment of thepresent invention;

FIG. 6 is a schematic cross-sectional view for showing an example of anoutline configuration of main parts (VA liquid crystal display panel)the liquid crystal display device according to the embodiment of thepresent invention;

FIG. 7 is a schematic view of an optical system for anchoringmeasurement studied in the embodiment of the present invention; and

FIG. 8 is a flowchart of manufacturing steps of the liquid crystaldisplay device using the alignment film according to the embodiment ofthe present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described in detail withreference to the drawings. It should be noted that constitutionalelements having the same functions are given the same reference numeralsin the all drawings for explaining an embodiment, and the explanationthereof will not be repeated.

FIG. 1 shows an outline view of a basic configuration of an alignmentfilm in a liquid crystal display device according an embodiment of thepresent invention. In the liquid crystal display device, an alignmentfilm 3 is formed on an underlayer 4, and a liquid crystal layer 5 isformed on the alignment film. Although not especially illustrated, anopposed substrate in which an alignment film having the sameconfiguration is formed is combined therewith. A layer 1 in which theratio of elements is changed towards the film thickness direction isformed on the surface of the alignment film 3 on the side of the liquidcrystal layer, and another layer 2 is formed under the layer 1. In thiscase, the film thickness direction is assumed as the z-direction, theuppermost position of the alignment film in contact with the liquidcrystal layer is assumed as z₀, the lowermost position of the layer 1 inwhich the ratio of elements is changed towards the film thicknessdirection is assumed as z₁, and the lower end of the layer 2 thereunderis assumed as z₂.

Each of FIG. 2 schematically shows a state in which the ratio of oxygenO atoms among the elements of the alignment film is changed towards thefilm thickness direction in the liquid crystal display device shown inFIG. 1. The range from z₀ to z₁ corresponds he layer 1. Each of FIG. 2Aand FIG. 2B shows a case in which the ratio is decreased from thesurface of the film and is increased later, and FIG. 2C shows a case inwhich the ratio is moderately decreased. As a difference between FIG. 2Aand FIG. 2B, FIG. 2A shows a case in which the ratio of oxygen atoms ofthe layer 2 is lower than that at the lower end of the layer 1, and FIG.2B shows a case in which the both ratios are the same. As describedabove, the elemental composition in the layer 1 can be changed in acomplicated manner. However, in order to obtain, excellent afterimagecharacteristics as will be described later, it is necessary to increasethe ratio of oxygen atoms on the surface of the alignment film whilemaintaining the hydrophobic state of the surface of the alignment film.A maximum value C_(max) and a minimum value C_(min) of the ratio ofoxygen atoms in the layer thickness direction are parameters tocharacterize the layer 1, and the ratio desirably becomes C_(max) whenz=z₀. It should be noted that some element has the alignment film formedusing only a layer in which the elemental composition is changed withoutproviding the layer 2. In this case, as a general configuration, atwo-layered structure as shown in FIG. 1 is illustrated.

Such changes in the elemental composition can be analyzed using variousthin-film surface analyses, such as X-ray photoemission spectroscopy(XPS), Auger electron spectroscopy, and a time-of-flight secondary ionmass spectrometry device (TOF-SIMS). First, a liquid crystal panel of atarget liquid crystal display device is decomposed. Then, the liquidcrystal is washed in an alkane solvent such as cyclohexane, and is driedto he used as a specimen for various analyses. Especially, in order toanalyze in the depth direction of the film thickness direction, thespecimen can be evaluated by various analyses while being sputteredusing gas ions such as Ar.

In order to increase the ratio of oxygen atoms on the surface of thealignment film, the film can be produced in accordance with thefollowing procedure. Specifically, the precursor of polyimide capable ofphoto-alignment is applied onto an underlayer to ran polyimide thin filmby heating, and polarized ultraviolet rays are irradiated on the surfaceof the thin film, so that an alignment restraining force is applied.Before, during, or after the irradiation of the polarized ultravioletrays, the surface of the thin film is exposed to an oxidationatmosphere, and thus a layer having a high ratio of oxygen atoms fromthe surface of the thin film to the inside can be formed. As a method ofoxidation process, ozone gas from the air by a source of ultravioletrays and various oxidizing agents (hydrogen peroxide water, hypochlorouswater, ozone water, hypoiodous acid water, permanganic acid water, andthe like) are used. In this case, the distribution in which the ratio ofoxygen atoms is changed from the surface of the thin film to the insidediffers depending on the oxidation atmosphere to be used or the exposureconditions. Further, in addition to their radiation of the polarizedultraviolet rays and the exposure to the oxidation atmosphere, drying byheating or irradiation of light having a different wavelength includinginfrared rays can be performed before, after, or during these processes.Alternatively, various solvent processes including water to removeforeign substances on the surface can be performed before or after theseprocesses. The layer having the increased ratio of oxygen atoms isdesirably formed on the surface of the photo-alignment film so as not todecrease the liquid crystal alignment restraining force by thephoto-alignment process. Specifically, the ratio of the layer to beformed is desirably half or smaller the film thickness from the surface,in contact with the liquid crystal, of the alignment film layer capableof photo-alignment. More desirably, the ratio is one tenth or smaller ofthe film thickness. Forming the layer having the increased and limitedratio of oxygen atoms on the surface of the photo-alignment film cansuppress harmful effects due to excessive oxidation on the surface ofthe alignment film caused by further increasing the ratio of oxygenatoms. For example, the following harmful effects can be suppressed: thesurface of the alignment film is changed to hydrophilicity to decreasethe contact angle relative to water by 20 degrees or more and the mutualaction between the alignment film and liquid crystal molecules ischanged. On the other hand, although the expression mechanism has notbeen clarified yet, the characteristics of retaining the liquid crystalalignment restraining force can be improved by photo-alignment. Forexample, even though the same liquid crystal alignment restraining forceis held immediately after the formation of the liquid crystal displaydevice, the liquid crystal layer is continued to be aligned for a longtime by an electric field in a direction different from the liquidcrystal alignment, direction in which the liquid crystal alignmentrestraining force is induced, and the afterimage time until beingreturned to the initial alignment direction after the electric field isremoved can be shortened.

Further, when producing the alignment film, the composition can beadjusted in such a manner that two or more types of alignment films areoverlapped with each other to be applied and imidized, or two or moretypes of polyimide precursors are blended to be applied and imidized.

The alignment films obtained after completion of such processes can beassembled to the liquid crystal display device by a general method.

Next, the liquid crystal display device in which the alignment films areproduced will be described. Each of FIG. 3A to FIG. 3D is a schematicview for showing an example of an outline configuration of the liquidcrystal display device according to the embodiment of the presentinvention. FIG. 3A is a schematic block diagram for showing an exampleof an outline configuration of the liquid crystal display device. FIG.3B is a schematic circuit diagram for showing an example of a circuitconfiguration of one pixel in the liquid crystal display panel. FIG. 3Cis a schematic plan view for showing an example of an outlineconfiguration of the liquid crystal display panel. FIG. 3D is aschematic cross-sectional view for showing an example of across-sectional configuration taken along the line A-A′ of FIG. 3C.

The alignment film in which the hydrophobic state is maintained and onthe surface of which the ratio of oxygen atoms is increased is appliedto, for example, an active matrix-type liquid crystal display device.The active matrix-type liquid crystal display device is used for, forexample, a display (monitor) for a portable electronic device, a displayfor a personal computer, a display for printing or designing, a displayfor a medical device, or a liquid crystal television.

For example, as shown in FIG. 3A, the active matrix-type liquid crystaldisplay device includes a liquid crystal display panel 101, a firstdriving circuit 102, a second driving circuit 103, a control circuit104, and a backlight 105.

The liquid crystal display panel 101 has plural scanning signal lines GL(gate lines) and plural video signal lines DL (drain lines). The videosignal lines DL are connected to the first driving circuit 102, and thescanning signal lines GL are connected to the second driving circuit103. It should be noted that FIG. 3A shows some of the plural scanningsignal lines GL, and more scanning signal lines GL are densely arrangedin the actual liquid crystal display panel 101. As similar to the above,FIG. 3A shows some of the plural video signal lines DL, and more videosignal lines DL are densely arranged in the actual liquid crystaldisplay panel 101.

Further, a display area DA of the liquid crystal display panel 101 isconfigured using a set of plural pixels, and an area occupied by onepixel in the display area DA corresponds to an area surrounded by, forexample, adjacent two scanning signal lines GL and adjacent two videosignal lines DL. In this case, the circuit of one pixel is configured asshown in, for example, FIG. 3B, and has a TFT element Tr functioning asan active element, a pixel electrode PX, a common electrode CT(occasionally referred to as an opposed electrode), and a liquid crystallayer LC. In this case, the liquid crystal display panel 101 is providedwith, for example, common lines CL that share common electrodes CT ofplural pixels.

Further, the liquid crystal display panel 101 is structured in such amanner that alignment films 606 and 705 are formed on the surfaces of anactive matrix substrate (TFT substrate) 106 and an opposed substrate107, respectively, and a liquid crystal layer LC (liquid crystalmaterial) is arranged between the alignment films as shown in, forexample, FIG. 3C and FIG. 3D. Further, although not especiallyillustrated, an intermediate layer (for example, an optical intermediatelayer such as a retardation film, a color conversion layer, or anoptical diffusion layer) may be appropriately provided each between thealignment film 606 and the active matrix substrate 106 and between thealignment film 705 and the opposed substrate 107.

In this case, the active matrix substrate 106 and the opposed substrate107 are allowed to adhere to each other using a circular seal material108 provided outside the display area DA, and the liquid crystal layerLC is sealed in a space surrounded by the alignment film 606 on the sideof the active matrix substrate 106, the alignment film 705 on the sideof the opposed substrate 107, and the seal material 108. In this case,the liquid crystal display panel 101 of the liquid crystal displaydevice having the backlight 105 has a pair of polarizing plates 109 aand 109 b that are arranged to face each other while sandwiching theactive matrix substrate 106, the liquid crystal layer LC, and theopposed substrate 107.

It should be noted that the active matrix substrate 106 is a substrateobtained by arranging the scanning signal lines GL, the video signallines DL, the active elements (TFT elements Tr), and the pixelelectrodes PX on an insulating substrate such as a glass substrate.Further, in the case where the driving method of the liquid crystaldisplay panel 101 is a horizontal electric field driving method such asan IPS method, the common electrodes CT and the common lines CL arearranged on the active matrix substrate 106. Further, in the case wherethe driving method of the liquid crystal display panel 101 is a verticalelectric field driving method such as a TN method or a VA (VerticallyAlignment) method, the common electrodes CT are arranged on the opposedsubstrate 107. In the case of the liquid crystal display panel 101 ofthe vertical electric field driving method, each of the commonelectrodes CT is generally a large-area plate electrode shared by allpixels, and no common lines CL are provided.

Further, in the liquid crystal display device according to theembodiment of the present invention, plural columnar spacers 110 areprovided in the space in which the liquid crystal layer LC is sealed toequalize, for example, the thickness (referred to as a cell gap in somecases) of the liquid crystal layer LC in each pixel. The plural columnarspacers 110 are provided on, for example, the opposed substrate 107.

The first driving circuit 102 is a driving circuit that generates avideo signal (referred to as gradation voltage in some cases) applied tothe pixel electrode PX of each pixel through the video signal lines DLand that is generally referred to as a source driver or a data driver.Further, the second driving circuit 103 is a driving circuit thatgenerates a scanning signal applied to the scanning signal lines DL andthat is generally referred to as a gate driver or a scanning driver.Further, the control circuit 104 is a circuit that controls theoperation of the first driving circuit 102, the operation of the seconddriving circuit 103, and the brightness of the backlight 105, and is acontrol circuit generally referred to as a TFT controller or a timingcontroller. Further, the backlight 105 is, for example, a fluorescentlight such as a cold cathode fluorescent light, or a light source suchas a light-emitting diode (LED). Light emitted from the backlight 105 isconverted into a planar light beam by a reflective plate, a light guideplate, an optical diffusion plate, and a prism sheet (all of which arenot shown) to be irradiated onto the liquid crystal display panel 101.

FIG. 4 is a schematic view for showing an example of an outlineconfiguration of an IPS liquid crystal display panel of the liquidcrystal display device according to the embodiment of the presentinvention. In the active matrix substrate 106, the scanning signal linesDL, the common lines CL (not shown), and a first insulating layer 602covering the same are formed on the surface of an insulating substratesuch as a glass substrate 601. On the first insulating layer 602, formedare a semiconductor layer 603 of the TFT element Tr, the video signallines DL, the pixel electrodes PX, and a second insulating layer 604covering the same. The semiconductor layer 603 is arranged above thescanning signal lines GL, and a part of the scanning signal lines GLlocated under the semiconductor layer 603 functions as a gate electrodeof the TFT element Tr.

Further, the semiconductor layer 603 is configured in such a mannerthat, for example, a source diffusion layer and a drain diffusion layercomposed of second amorphous silicon that is different in the type orconcentration of impurities from first amorphous silicon are laminatedon an active layer (channel formation layer) composed of first amorphoussilicon. In this case, a part of the video signal lines DL and a part ofthe pixel electrodes PX are overlapped with the semiconductor layer 603,and the parts overlapped with the semiconductor layer 603 function as adrain electrode and a source electrode of the TFT element Tr.

Incidentally, the source and the drain of the TFT element Tr areswitched to each other depending on the relation of bias, namely, thehigh-low relation between the electric potential of the pixel electrodesPX and the electric potential of the video signal lines DL when the TFTelement Tr is turned on. However, in the following description of thespecification, the electrode connected to the video signal lines DL isreferred to as a drain electrode, and the electrode connected to thepixel electrodes is referred to as a source electrode. On the secondinsulating layer 604, formed is a third insulating layer 605 (organicpassivation film) with the surface flattened. On the third insulatinglayer 605, formed are the common electrodes CT and the alignment film606 covering the common electrodes and the third insulating layer 605.

The common electrodes CT are connected to the common lines CL through acontact hole (through-hole) that penetrates the first insulating layer602, the second insulating layer 604, and the third insulating layer605. Further each of the common electrodes CT is formed so that, forexample, a gap Pg between the common electrode CT and the pixelelectrode PX on a plane is about 7 μm. A polymeric material described hefollowing examples is applied to the alignment film 606, and a surfaceprocess (photo-alignment and an oxidation process are performed toprovide a liquid crystal alignment capabilty the surface, so that theratio of oxygen atoms on the surface of the alignment film is while thehydrophobicity maintained.

On the other hand, opposed substrate 107, a black matrix 702, a colorfilter (703R, 703G, and 703B), an overcoat layer 704 covering the sameare formed on the surface of the insulating substrate such as the glasssubstrate 701. The black matrix 702 is, example, a grid light-blockingfilm to provide the display area DA with an opening area of each pixel.Further, the color filter (703R, 703G, and 703B) is, for example, a filmthat allows only light with a specific wavelength area (color) of whitelight from the backlight 105 to pass through. In the case where theliquid crystal display device is adapted to RGB color display, the colorfilter 703R that allows red light pass through, the color filter 703Gthat allows green light to pass through, and the color filter 703B thatallows blue light to pass through are arranged (a pixel of one color isrepresentatively shown in this case).

Further, the surface of the overcoat layer 704 is flattened. On theovercoat layer 704, formed are the plural columnar spacers 110 and thealignment film 705. Each of the columnar spacers 110 is formed in, forexample, a conical trapezoidal shape (referred to as a trapezoidalrotator in some cases) with a flat top, and is formed at a positionoverlapped with a part of the scanning signal lines GL of the activematrix substrate 106 except a part where the TFT element Tr is arrangedand a part crossing the video signal lines DL. Further, the alignmentfilm 705 is formed using, for example, polyimide-based resin, and asurface process (photo-alignment process) and an oxidation process areperformed to provide a liquid crystal alignment capability on thesurface, so that the ratio of oxygen atoms on the surface of thealignment film is increased while the hydrophobicity is maintained.

Further, liquid crystal molecules 111 of the liquid crystal layer LC inthe liquid crystal display panel 101 of the system shown in FIG. 4 arealigned substantially in parallel with the surfaces of the glasssubstrates 601 and 701 when the pixel electrodes PX and the commonelectrodes CT are the same in electric potential, namely, when noelectric field is applied, and are homogeneously aligned towards theinitial alignment direction regulated by an alignment restraining forceprocess performed on the alignment films 606 and 705. When the TFTelement Tr is turned on and gradation voltage applied to the videosignal lines DL is written into the pixel electrodes PX to cause anelectric potential difference between the pixel electrodes PX and thecommon electrodes CT, electric fields 112 (lines of electric force) asshown in the drawing are generated, and the electric fields 112 with anintensity corresponding to the electric potential difference between thepixel electrodes PX and the common electrodes CT are applied the liquidcrystal molecules 111.

In this case, the direction of the liquid crystal molecules 111configuring the liquid crystal layer LC is changed to the direction ofthe electric fields 112 due to the mutual action between the dielectricanisotropy of the liquid crystal layer LC and the electric fields 112,and thus the refraction anisotropy of the liquid crystal layer LC ischanged. Further, in this case, the direction of the liquid crystalmolecules 111 is determined on the basis of the intensity (magnitude ofthe electric potential difference between the pixel electrodes PX andthe common electrodes CT) of the electric fields 112 to be applied.Thus, the liquid crystal display device can display video and images insuch a manner that, for example, the electric potential of the commonelectrodes CT is fixed, and the gradation voltage applied to the pixelelectrodes PX is controlled for each pixel to change the lighttransmittance of each pixel.

FIG. 5 is a schematic view for showing an example of an outlineconfiguration of an FFS liquid crystal display panel of another liquidcrystal display device according to the embodiment of the presentinvention. In the active matrix substrate 106, the common electrodes CT,the scanning signal lines GL, the common lines CL, and the firstinsulating layer 602 covering the same are formed on the surface of theinsulating substrate such as the glass substrate 601. On the firstinsulating layer 602, formed are the semiconductor layer 603 of the TFTelement Tr, the video signal lines DL, a source electrode 607, and thesecond insulating layer 604 covering the same. In this case, a part ofthe video signal lines DL and a part of the source electrode 607 areoverlapped with the semiconductor layer 603, and the parts overlappedwith the semiconductor layer 603 function as the drain electrode and thesource electrode of the TFT element Tr.

Further, in the liquid crystal display panel 101 of FIG. 5, the thirdinsulating layer 605 is not formed, but the pixel electrodes PX and thealignment film 606 covering the pixel electrodes PX are formed on thesecond insulating layer 604. Although not shown in the drawing, thepixel electrodes PX are connected the source electrode 607 through acontact hole (through-hole) that penetrates the second insulating layer604. In this case, the common electrode CT formed on the surface of theglass substrate 601 is formed in a plate shape at an area (opening area)surrounded by adjacent two scanning signal lines DL and adjacent twovideo signal lines DL, and the pixel electrode PX having plural slits islaminated on the plate-like common electrode CT. Further, in this case,the common electrodes CT of the pixels arranged in the extensiondirection of the scanning signal lines CL are shared by the common linesCL. On the other hand, the opposed substrate 107 in the liquid crystaldisplay panel 101 of FIG. 5 has the same configuration as that of theopposed substrate 107 of the liquid crystal display panel 101 of FIG.3D. Therefore, a detailed explanation related to the configuration ofthe opposed substrate 107 will be omitted.

FIG. 6 is a schematic cross-sectional view for showing an example of across-sectional configuration of main parts of a VA liquid crystaldisplay panel of another liquid crystal display device according to theembodiment of the present invention. In the liquid crystal display panel101 of the vertical electric field driving method, for example, thepixel electrodes PX are formed on the active matrix substrate 106, andthe common electrodes CT are formed on the opposed substrate 107 asshown in FIG. 6. In the case of the VA liquid crystal display panel 101as one of the vertical electric field driving methods, for example, thepixel electrodes PX and the common electrodes CT are formed in a flatshape (simple plate shape) using a transparent conductor such as ITO.

In this case, the liquid crystal molecules 111 are arranged vertical tothe surfaces of the glass substrates 601 and 701 by the alignment films606 and 705 when the pixel electrodes PX and the common electrodes CTare the same in electric potential, namely, when no electric field isapplied. Then, when an electric potential difference between the pixelelectrodes PX and the common electrodes CT is generated, the electricfields 112 (lines of electric force) that are substantially vertical tothe glass substrates 601 and 701 are generated, and the liquid crystalmolecules 111 lean in the direction parallel to the substrates 601 and701 to change the polarization state of incident light. In this case,the direction of the liquid crystal molecules 111 is determined on thebasis of the intensity of the electric fields 112 to be applied.

Thus, the liquid crystal display device displays video and images insuch a manner that, for example, the electric potential of the commonelectrodes CT is fixed, and the video signal (gradation voltage) appliedto the pixel electrodes PX is controlled for each pixel to chance thelight transmittance of each pixel. Further, as the configuration of thepixel in the VA liquid crystal display panel 101, for example, as theplane shapes of the TFT elements Tr and the pixel electrodes PX, thereare various configurations. Thus, the configuration of the pixel in theVA liquid crystal display panel 101 shown in FIG. 6 may be any one ofthe configurations. In this case, a detailed explanation related to theconfiguration of the pixel in the liquid crystal display panel 101 willbe omitted. It should be noted that the reference numeral 608 denotes aconductive layer, the reference numeral 609 denotes a protrusionformation member, the reference numeral 609 a denotes a semiconductorlayer, and the reference numeral 609 b denotes a conductive layer.

The embodiment of the present invention relates to the liquid crystaldisplay panel 101, especially, a part in contact with the liquid crystallayer LC in the active matrix substrate 106 and the opposed substrate107 in the above-described active matrix liquid crystal display deviceand a surrounding configuration. Therefore, detailed explanations forthe configurations of the first driving circuit 102, the second drivingcircuit 103, the control circuit 104, and the backlight 105 to which theconventional technique can be applied as it is will be omitted.

In order to manufacture these liquid crystal display devices, variousalignment film materials, alignment processing methods, and variousliquid crystal materials already used for the liquid crystal displaydevice can be used, and can be applied to various processes performedwhen assembling and processing the liquid crystal display device. Anexample thereof is shown in FIG. 8. First, an active matrix substrateand an opposed substrate are prepared through manufacturing processes,and the surface of an underlayer forming an alignment film is cleanedusing various surface processing methods such as a UV/ozone method, anexcimer UV method, and an oxygen plasma method. Next, the precursor ofthe alignment film is applied using various printing methods such asscreen printing, flexographic printing, and ink-jet printing, and aleveling process is performed to have a uniform film thickness under apredetermined condition. Then, for example, the film is heated at atemperature of 180° C. or higher, and polyamide as the precursor is putto imidization reaction to polyimide. Further, polarized ultravioletrays are irradiated and a proper post-process is performed using desiredmeans, so that an alignment restraining force is generated on thesurface of the polyimide alignment film (photo-alignment). At the stageof the irradiation of the polarized ultraviolet rays or the processafter the irradiation, heating or irradiation of light with anotherwavelength can be performed. Further, at the stage before or after theirradiation of the polarized ultraviolet rays, the above-describedprocedure of the exposure to the oxidation atmosphere is added, so thatthe photo-alignment film whose surface has a high ratio of oxygen atomsis formed in a state where hydrophobicity maintained. The active matrixsubstrate with the alignment film thus formed and the opposed substrateare pasted together while keeping a certain gap, so that the directionof the alignment restraining force is oriented in a desired direction.After a predetermined period of time, a portion in the gap is filledwith liquid crystal, and an end portion of the substrate is sealed, sothat a liquid crystal panel is completed. Then, optical films such as apolarizing plate and a retardation film are pasted to the panel, and adriving circuit, a backlight, and the like are attached, so that theliquid crystal display device can be obtained. It should be noted thatthe both of the alignment film formed on the active matrix substrate(TFT substrate) and the alignment film formed on the opposed substrate(CF substrate) are exposed to an oxidation atmosphere in theabove-described description. However, even if any one of them is exposedto an oxidation atmosphere, improved effects for afterimagecharacteristics cart be obtained. It is obvious that the afterimagecharacteristics can be further improved by performing the oxidationprocess for the both.

Next, the anchoring force of liquid crystal representing the magnitudeof the alignment restraining force can be measured by the followingmethod. Specifically, a pair of two glass substrates is coated with thealignment films to perform a photo-alignment process, and spacers havingan appropriate thickness d are provided between the substrates, so thatthe alignment directions of the two alignment films are made parallel toeach other. Accordingly, homogeneous alignment liquid crystal cells forevaluation can be produced. Each cell is sealed with a nematic liquidcrystal material (helix pitch p and elastic constant K₂) with a chiralagent whose material physical property is already known, and the cellsfor evaluation are once held in the liquid crystal isotropic phase inorder to stabilize the alignment. Then, the temperature is returned to aroom temperature to measure a twist angle Ø₂ the following method.

Next, most liquid crystal in each cell is removed by air pressure or thecentrifugal force, and the inside of each cell is washed by solvent anddried. Then, each cell is sealed with the same liquid crystal without achiral agent, and the alignment is similarly stabilized to measure atwist angle Ø₁. In this case, the anchoring intensity can be obtained bythe following equation.

$\begin{matrix}{A_{\varphi} = \frac{2\; {K_{2}\left( {{2\; \pi \; {d/p}} - \varphi_{2}} \right)}}{d\; {\sin \left( {\varphi_{2} - \varphi_{1}} \right)}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Further, the twist angle was measured using an optical system as in FIG.7. Specifically, a visible light source 6 and a photomultiplier tube 10are collimated on the same straight line, and a polarizer 7, a cell forevaluation 8, and an analyzer 9 are arranged therebetween in this order.A tungsten lamp is used for the visible light source 6. First, thetransmission axis of the polarizer 7 and the absorption axis of theanalyzer 9 are set so as to be substantially parallel to the alignmentdirection (L-L′) of the alignment film of the cell for evaluation 8.Next, only the polarizer is rotated, and the angle is changed so thatthe intensity of transmitted light is minimized. Next, only the analyzeris rotated, and the angle is changed so that the intensity oftransmitted light is minimized. Thereafter, the rotation of only thepolarizer and the rotation of only the analyzer are repeated as similarto the above until the angle is uniformed. The transmission axisrotational angle Ø_(polarizer) of the polarizer and the absorption axisrotational angle Ø_(analyzer) of the analyzer at the time of beingfinally converged are defined as “twist angleØ=Ø_(analyzer)−Ø_(polarizer). In this case, a read error in themeasurement can be reduced by adjusting the refractive index anisotropyΔn of liquid crystal to be used and the thickness of the liquid crystalcell.

Next, a method of determining a brightness relaxation constant will bedescribed below. In accordance with the procedure as described above indetail, various liquid crystal display devices including the alignmentfilms are produced. A black/white window pattern is continuouslydisplayed on the liquid crystal display device for a predeterminedperiod of time (this is referred to as a printing time), and isimmediately switched to the display voltage of the grey level asgradation on the entire screen to measure the time when the windowpattern (referred to as printing or afterimage) disappears.

Ideally, no residual electric charge is generated in any part of theliquid crystal display device and the direction of the alignmentrestraining force is not disturbed in the alignment film. Thus,immediately after being switched to the display voltage, the grey levelis displayed on the entire screen. However, the effective alignmentstate of a bright area (white pattern portion) becomes different from anideal level due to the generation of the residual electric charge alongwith the driving and the disturbance of the direction of the alignmentrestraining force, and thus the brightness is differently viewed.Holding the display for a much longer time at the voltage of thegradation display results in the residual electric charge and thedirection of the alignment restraining force at the voltage, and thusthe display can be uniformly viewed. The in-plane brightnessdistribution of the liquid crystal display element was measured using aCCD camera, and the brightness relaxation constant of the liquid crystaldisplay element was obtained at the printing time that was a time untilthe display was uniformly viewed. It should be noted that if thebrightness was not relaxed after 480 hours, the evaluation wasterminated to be described as ≥400.

Hereinafter, the present invention will be described in more detailsusing embodiments. However, the technical scope of the present inventionis not limited to the following embodiments.

First Embodiment

First, a liquid crystal display device which had a layer with the ratioof elements configuring an alignment film changed towards the filmthickness direction on the surface of the alignment film and in whichthe ratio of oxygen atoms on the surface of the alignment film washigher than that in the alignment film was produced, and resultsobtained by comparing anchoring characteristic and afterimagecharacteristics with each other will be described using the drawings andtables.

Three kinds of substrates such as an alkali-free glass (AN-100 of ASAHIGLASS CO., LTD.) substrate, a substrate on which a tin-doped indiumoxide (ITO) thin-film was formed by sputtering, and a substrate on whicha silicon nitride (SiNx) thin-film was formed were used. Each of thebase substrates thus prepared was washed by a chemical such as a neutraldetergent, and then each surface was cleaned by a UV/O₃ process beforethe precursor of the alignment film was applied. The followings wereused as alignment films for tests. As the component of a first alignmentfilm, the chemical structure such as (Chemical formula 3) was selectedfor the framework of polyamide acid as the precursor of polyimide of(Chemical formula 2).

In accordance with an existing chemical synthetic method, polyamide acidwas synthesized using dianhydride and diamine as raw materials. Further,as the component of a second alignment film, (Chemical formula 4) wasselected.

The molecular weights of polyamide acid were obtained using themolecular weights in terms of polystyrene by a GPC (gel permeationchromatograph analysis), and were 16000 and 14000. The first alignmentfilm and the second alignment film were dissolved at a ratio of 1 to 1in a mixed solvent of butyl cellosolve, N-methylpyrrolidone, andγ-butyrolactone. The resultants were formed on predetermined basesubstrates by flexo printing to obtain thin films, and the thin filmswere temporarily dried at a temperature of 40° C. or higher, followed byimidization in a bake furnace at a temperature of 150° C. or higher. Theconditions of forming the thin films were adjusted in advance so thatthe film thickness in this case became approximately 100 nm. Next, inorder to provide a liquid crystal alignment restraining force by cuttinga part of the molecular framework of a highly-polymerized compound withpolarized light, polarized ultraviolet rays (dominant wavelength of 280nm) were collected and irradiated using an ultraviolet ray lamp(low-pressure mercury lamp), a wire grid polarizer, and an interferencefilter. In this case, one film which was photo-aligned while ozone gasgenerated around the ultraviolet ray lamp was forcibly blown, and theother film which was obtained by normally irradiating only ultravioletrays were produced. After a predetermined period of time, foreignsubstances on the surfaces were removed by washing by pure water anddrying by heating to produce alignment film specimens. Further, theelemental compositions of the obtained alignment films were measured byan XPS method. As an X-ray photoemission spectrroscopy device, AXIS-HSof Shimadzu Corporation/Kratos was used. The measurement conditions wereas follows X-ray source monochrome Al (tube voltage of 15 kV and tubecurrent of 15 mA), lens condition Hybrid (analysis area of 600×1000 μm),resolution Pass Energy 40, and scanning speed of 20 eV/min (0.1 eVstep). The elemental composition from the surface to the depth directionwas analyzed by sputtering using A^(r)+ion. The evaluation results areshown in Table 1.

TABLE 1 z (nm) C (%) N (%) O (%) (a) 0 68.8 6.8 24.4 10 74.0 7.6 18.4 2073.1 7.1 19.8 30 74.2 7.2 18.6 40 73.2 7.7 19.1 50 74.8 8.5 16.7 60 75.410.1 14.5 70 76.2 10.4 13.4 80 76.1 10.0 13.9 90 75.9 10.4 13.7 100 76.710.2 13.1 (b) 0 74.9 7.5 17.6 10 74.2 7.5 18.3 20 73.5 7.1 19.4 30 74.17.1 18.8 40 74.1 7.5 18.4 50 75.3 8.5 16.2 60 75.9 10.7 13.4 70 75.410.3 14.3 80 75.9 10.7 13.4 90 75.9 10.0 14.1 100 76.4 10.0 13.6

Table 1 shows changes in the depth direction (z-direction) of theelemental compositions of the obtained films. In this case, the resultsfor the one film to which ozone gas was blown were shown in Table 1(a),and the results for the other film to which no ozone gas was blown wereshown in Table 1(b). In the case of the film to which no ozone gas wasblown, the composition ratio of elements configuring the alignment filmin terms of carbon C, nitrogen N, and oxygen O was as follows: C=74 to75%, N=7%, and O=17 to 19% when z=0 to 40 nm; and O=75 to 76%, N=10%,and O=13 to 14% when z=60 to 100 nm. In this case, in the case of thefirst alignment film alone, C=74.1%, N=7.4%, and O=18.5%. In the case ofthe second alignment film alone, C=75.6%, N=10.3%, and O=13.8%. Thefirst alignment film and the second alignment film were separated intotwo layers at a mixture ratio of 1 to 1 in the film thickness direction.On the contrary, in the case of the film to which ozone gas was blown,the composition ratio was as follows: C=69%, N=7%, and O=24% when z=0nm; C=73 to 74%, N=7%, and O=18 to 19% when z=10 to 40 nm; and C=75 to76%, N=10%, and O=13 to 14% when z=60 to 100 nm. The results show thatthe ratio of oxygen O was increased only on the outermost layer whereasthe ratio of carbon C was relatively decreased. It can be understoodthat the ratio of oxygen on the outermost layer was increased by about26% ((24−19)÷19=0.26) relative to the first alignment film as comparedto the inside of the film. It should be noted that the both films werehydrophobic irrespective of blowing of ozone.

The anchoring energy was measured using each of the alignment films. Theanchoring energy of the film to which no ozone was blown was 2.0 mJ/m²where as that of the film to which ozone was blown was 2.4 mJ/m². Thus,the anchoring characteristic was improved.

Further, an IPS liquid crystal display device was produced using each ofthe alignment films to measure the brightness relaxation constant. Thebrightness relaxation constant of the film to which no ozone was blownwas 54 hours whereas that of the film to which ozone was blown was 42hours. Thus, the brightness relaxation constant was improved.

As described above, if the liquid crystal display device which had alayer with the ratio of elements configuring the alignment film changedtowards the film thickness direction on the surface of the alignmentfilm and in which the ratio of oxygen atoms on the surface of thealignment film was higher than that in the alignment film was producedusing ozone gas when the photo-alignment process was performed, it wasfound that the anchoring characteristics and afterimage characteristicswere improved.

According to the embodiment, even in the case of using thephoto-alignment technique, it is possible to provide a liquid crystaldisplay device and a manufacturing method thereof in which excellentafterimage characteristics can be stably obtained.

Second Embodiment

Next, a liquid crystal display device which had a layer with the ratioof elements configuring an alignment film changed towards the filmthickness direction on the surface of the alignment film and in whichthe ratio of oxygen atoms on the surface of the alignment film washigher than that in the alignment film was produced under differentproducing conditions, and results obtained by comparing anchoringcharacteristics and afterimage characteristics with each other will bedescribed using the drawings and tables.

As the material of the alignment films, the same material as Embodiment1 was used. Under the same producing conditions, the substrates werecoated with the alignment films, and were burned for imidization. Then,using the same polarized ultraviolet ray source, the alignment processwas performed without blowing ozone gas. After a predetermined period oftime, foreign substances on the surfaces were removed by washing by purewater and drying by heating (these processes were the same as those forthe alignment films shown as the comparison of Embodiment 1). The thinfilms were dipped into hydrogen peroxide water (3%) for one minute, andforeign substances on the surfaces were removed by washing by pure waterand drying by heating again to produce alignment film specimens. Theevaluation results are shown in Table 2.

TABLE 2 z (nm) C (%) N (%) O (%) 0 69.1 7.2 23.7 10 71.2 7.2 21.6 2072.9 7.2 19.9 30 72.9 7.3 19.8 40 73.9 7.3 18.8 50 75.4 8.5 16.1 60 75.310.3 14.4 70 76.7 10.2 13.1 80 76.1 10.6 13.3 90 75.5 10.8 13.7 100 75.610.8 13.6

Table 2 shows changes in the depth direction (z-direction) of theelemental compositions of the obtained films. In the case of the filmfor which such processes were performed, the composition ratio ofelements configuring the alignment film in terms of carbon C, nitrogenN, and oxygen O was as follows: C=69%, N=7%, and O=24% when z=0 nm; andC=71%, N=7%, and O=22% when z=10 nm. However, C=73 to 74%, N=7%, andO=18 to 19% when z=20 to 40 nm, and C=75 to 76%, N=10%, and O=13 to 14%when z=60 to 100 nm. The results show that the ratio of oxygen O wasincreased only at an area near the outermost layer whereas the ratio ofcarbon C was relatively decreased. It can be understood that the ratioof oxygen on the outermost layer was increased by about 26%((24−19)÷19=0.26) relative to the first alignment film as compared tothe inside of the film. It should be noted that the. alignment filmsproduced in the embodiment exhibited hydrophobicity.

The anchoring energy was measured using each of the alignment films. Theanchoring energy of the film for comparison was 2.0 mJ/m where as thatprocessed with hydrogen peroxide water was 2.7 mJ/m². Thus, theanchoring characteristic was improved.

Further, an IPS liquid crystal display device was produced using each ofthe alignment films to measure the brightness relaxation constant. Thebrightness relaxation constant of the film for comparison was 54 hourswhereas that processed with hydrogen peroxide water was 36 hours. Thus,the brightness relaxation constant was improved.

As described above, if the liquid crystal display device which had alayer with the ratio of elements configuring the alignment film changedtowards the film thickness direction on the surface of the alignmentfilm and in which the ratio oxygen atoms on the surface of the alignmentfilm was higher than that in the alignment film was produced by anoxidation process using hydrogen peroxide water after thephoto-alignment process was performed, it was found that the anchoringcharacteristics and afterimage characteristics were improved.

According to the embodiment, even in the case of using thephoto-alignment technique, it is possible to provide a liquid crystaldisplay device and a manufacturing method thereof in which excellentafterimage characteristics can be stably obtained.

Third Embodiment

Next, a liquid crystal display device which had a layer with the ratioof elements configuring an alignment film changed towards the filmthickness direction on the surface of the alignment film and in whichthe ratio of oxygen atoms on the surface of the alignment film washigher than that in the alignment film was produced under differentproducing conditions, and results obtained by comparing anchoringcharacteristics and afterimage characteristics with each other will bedescribed using the drawings and tables.

As the material of the alignment films, the same material as Embodiment1 was used. Under the same producing conditions, the substrates werecoated with the alignment films, and were burned for imidization. Then,using the same polarized ultraviolet ray source, the alignment processwas performed without blowing ozone gas. After a predetermined period oftime, foreign substances on the surfaces were removed by washing by purewater and drying by heating (these processes were the same as those forthe alignment films shown as the comparison of Embodiment 1). The thinfilms were dipped into ozone water (1 ppm) for one minute, and foreignsubstances on the surfaces were removed by washing by pure water anddrying by heating again to produce alignment film specimens. Theevaluation results are shown in Table 3.

TABLE 3 z (nm) C(%) N (%) O (%) 0 68.4 7.2 24.4 10 69.8 7.1 23.1 20 71.26.9 21.9 30 71.9 7.4 20.7 40 72.5 7.4 20.1 50 74.7 9.0 16.3 60 75.6 10.314.1 70 76.1 10.1 13.8 80 76.2 10.3 13.5 90 75.9 9.9 14.2 100 75.9 10.114.0

Table 3 shows changes in the depth direction (z-direction) of theelemental compositions of the obtained films. In the case of the filmfor which such processes were performed, the composition ratio ofelements configuring the alignment film in terms of carbon C, nitrogenN, and oxygen O was as follows: C=68%, N=7%, and O=24% when z=0 nm; C=70to 73%; N=7%, and O=23 to 20% when z=10 to 40 nm; and C=75 to 76%,N=10%, and O=13 to 14% when z=60 to 100 nm. The results show that theratio of oxygen O was increased only at an area near the outermost layerwhereas the ratio of carbon C was relatively decreased. It should benoted that the alignment films produced in the embodiment exhibitedhydrophobicity.

The anchoring energy was measured using each of the alignment films. Theanohoring energy of the film for comparison was 2.0 mJ/m² where as thatprocessed with ozone water was 3.0 mJ/m². Thus, the anchoringcharacteristic was improved.

Further, an IPS liquid crystal display device was produced using each atthe alignment films to measure the brightness relaxation constant. Thebrightness relaxation constant of the film for comparison was 54 hourswhereas that processed with ozone water was 30 hours. Thus, thebrightness relaxation constant was improved.

As described above, if the liquid crystal display device which had alayer with the ratio of elements configuring the alignment film changedtowards the film thickness direction on the surface of the alignmentfilm and in which the ratio of oxygen atoms on the surface of thealignment film was higher than that in the alignment film was producedby an oxidation process using ozone water after the photo-alignmentprocess was performed, it was found that the anchoring characteristicsand afterimage characteristics were improved.

According to the embodiment, even in the case of using thephoto-alignment technique, it is possible to provide a liquid crystaldisplay device and a manufacturing method thereof in which excellentafterimage characteristics can be stably obtained.

Fourth Embodiment

Next, a liquid crystal display device which had a layer with the ratioof elements configuring an alignment film changed towards the filmthickness direction on the surface of the alignment film and in whichthe ratio of oxygen atoms on the surface of the alignment film washigher than that in the alignment film was produced under differentproducing condition, and results obtained by comparing anchoringcharacteristics and afterimage characteristics with each other will bedescribed using the drawings and tables.

As the material of the alignment films, the same material as Embodiment1 was used. Under the same producing conditions, the substrates werecoated with the alignment films, and were burned for imidization. Then,using the same polarized ultraviolet ray source, the alignment processwas performed without blowing ozone gas. After a predetermined period oftime, foreign substances on the surfaces were removed by washing by purewater and drying by heating (these processes were the same as those forthe alignment films shown as the comparison of Embodiment 1). The thinfilms were dipped into hypochlorous water (20 ppm) for 30 seconds, andforeign substances on the surfaces were removed by washing by pure waterand drying by heating again to produce alignment film specimens. Theevaluation results are shown in Table 4.

TABLE 4 z (nm) C (%) N (%) O (%) 0 68.2 6.5 25.3 10 68.5 7.2 24.3 2069.9 6.7 23.4 30 68.8 6.7 24.5 40 70.0 6.8 23.2 50 74.7 9.0 16.3 60 75.710.6 13.7 70 76.2 10.3 13.5 80 75.7 10.0 14.3 90 76.4 10.3 13.3 100 76.010.2 13.8

Table 4 shows changes in the depth direction (z-direction) of theelemental compositions of the obtained films. In the case of the filmfor which such processes were performed, the composition ratio ofelements configuring the alignment film in terms of carbon C, nitrogenN, and oxygen O was as follows: C=68%, N=7%, and O=25% when z=0 nm; C=68to 70%, N=7%, and O=24 to 23% when z=10 to 40 nm; and C=75 to 76%,N=10%, and O=13 to 14% when z=60 to 100 nm. The results show that theratio of oxygen O was increased only at an area near the outermost layerwhereas the ratio of carbon C was relatively decreased. It should benoted that the alignment films produced in the embodiment exhibitedhydrophobicity.

The anchoring energy was measured using each of the alignment films. Theanchoring energy of the film for comparison was 2.0 mJ/m² where as thatprocessed with hypochlorous water was 3.5 mJ/m². Thus, the anchoringcharacteristic was improved.

Further, an IFS liquid crystal display device was produced using each ofthe alignment films to measure the brightness relaxation constant. Thebrightness relaxation constant of the film for comparison was 54 hourswhereas that processed with hypochlorous water was 31 hours. Thus, thebrightness relaxation constant was improved.

As described above, if the liquid crystal display device which had alayer with the ratio of elements configuring the alignment film changedtowards the film thickness direction on the surface of the alignmentfilm and in which the ratio of oxygen atoms on the surface of thealignment film was higher than that in the alignment film was producedby an oxidation process using hypochlorous water after thephoto-alignment process was performed, it was found that the anchoringcharacteristics and afterimage characteristics were improved.

According to the embodiment, even in the case of using thephoto-alignment technique, it is possible to provide a liquid crystaldisplay device and a manufacturing method thereof in which excellentafterimage characteristics can be stably obtained.

Fifth Embodiment

Next, results obtained when the state of the oxidation process waschanged by changing the surface treatment time of the alignment filmsusing the method shown in Embodiment 1 will be described using thedrawings and tables.

As the material of the alignment films, the same material as Embodiment1 was used. Under the same producing conditions, the substrates werecoated with the alignment films, and were burned for imidization. Then,using the same polarized ultraviolet ray source, the alignment processwas performed by blowing ozone gas. After predetermined irradiation wascompleted, only ultraviolet rays were shut by a shutter, and only ozonegas was continued to expose to make longer the oxidation time for thesurfaces of the alignment films. The processes thereafter were the sameas those of Embodiment 1, and the oxygen concentration on the surfacesof the alignment films and the contact angle relative to water weremeasured. Further, the liquid crystal display device was similarlyassembled, and the brightness relaxation constant was measured. Theresults were shown in Table 5.

TABLE 5 CONTACT SURFACE OUTERMOST ANGLE OF BRIGHTNESS TREATMENT LAYERWATER RELAXATION CONDITION 0(%) (DEGREE) (TIME) INITIAL STAGE 17.6 56 54OZONE 0 24.4 53 42 GAS 5 24.9 49 35 PROCESSING 10 25.3 46 26 TIME 1525.8 43 25 AFTER 20 26.4 40 33 UV 25 26.8 38 41 (MINUTE) 30 27.2 35 48

In this case, the surface treatment condition described as the initialstage shows an alignment film when polarized ultraviolet rays wereirradiated without blowing ozone gas. The results show that the ratio ofoxygen atoms on the outermost layer was increased from 17.6% in theinitial stage to 24.4%, which was the same as the results shown inEmbodiment 1. However, it can be understood that if ozone gas wascontinued to be blown even after the polarized ultraviolet rays wereirradiated, the ratio of oxygen atoms on the outermost layer wascontinued to be increased with time. Further, in the case of the contactangle relative to water, the contact angle of 56 degrees in the initialstage was decreased as the blowing time of ozone gas became longer, andwas decreased to 35 degrees by 20 degrees or more in 30 minutes. On theother hand, the brightness relaxation constant was gradually decreasedfrom 54 hours in the initial stage. However, the brightness relaxationtime became longer as the processing time became longer after a minimumvalue of 25 hours in a processing time of 15 minutes. It was found thateven if the brightness relaxation constant was improved by increasingthe ratio of oxygen atoms on the surface of the photo-alignment film,the brightness relaxation constant was deteriorated if the ratio ofoxygen atoms was extremely increased, and the contact angle within theeffective processing time ranged to 14 degrees or smaller from theinitial stage. Thus, as the hydrophobic scale of the alignment film, itis advantageous that the contact angle of water is 38 degrees or more.Further, 40 degrees or more is desirable, and 43 degrees or more is moredesirable.

When the anchoring energy and brightness relaxation constant wereevaluated in each process of Embodiments 1 to 3 so that the contactangle of water on the surface of the alignment film became 43 degrees,excellent results could be obtained. Further, as a result of applyingthe alignment film to the liquid crystal display device, excellentafterimage characteristics could be obtained.

As described above, if the liquid crystal display device which had alayer with the ratio of elements configuring the alignment film changedtowards the film thickness direction on the surface of the alignmentfilm and in which the ratio of oxygen atoms on the surface of thealignment film was higher than that in the alignment film was producedby adding an oxidation process using ozone gas after the photo-alignmentprocess was performed, it was found that the anchoring characteristicsand afterimage characteristics were improved, but excessive oxidationadversely deteriorated the display performance of the liquid crystaldisplay device.

According to the embodiment, even in the case of using thephoto-alignment technique, it is possible to provide a liquid crystaldisplay device and a manufacturing method thereof in which excellentafterimage characteristics can be stably obtained. It should be notedthat it is advantageous that the contact angle of water on the alignmentfilm as the hydrophobic scale is 38 degrees or more.

It should be noted that the present invention is not limited to theabove-described embodiments, but various modified embodiments may beincluded. For example, the above-described embodiments have beendescribed in detail to understandably explain the present invention, andare not necessarily limited to those having the all configurationsdescribed above. Further, a part of the configuration in one embodimentcan be replaced by a configuration of another embodiment, and theconfiguration in one embodiment can be added to another embodiment. Inaddition, a part of the configuration in each embodiment can be added toor replaced by another, or deleted.

What is claimed is:
 1. A liquid crystal display device comprising: a thin-film transistor (TFT) substrate which includes a base substrate, pixel electrodes and TFTs and on which an alignment film is formed on a pixel; and an opposed substrate which is arranged to face the TFT substrate, wherein liquid crystal molecules are sandwiched between the alignment film of the TFT substrate and that of the opposed substrate, the alignment film is a polyimide material that can provide a liquid crystal alignment restraining force by irradiating polarized light, the polyimide material is made from a polyamide acid precursor or a polyamide acid ester precursor, the liquid crystal molecules are aligned in parallel with the base substrate, the ratio of oxygen atoms at a position with the highest oxygen concentration is 25% or higher than at a position with the lowest oxygen concentration in the alignment film, the ratio of oxygen atoms of the alignment film is gradually decreased from the surface contacting with the liquid crystal molecules towards the inside of the alignment in an alignment film thickness direction, and the alignment film is a photo degradation-type photo-alignment.
 2. The liquid crystal display device according to claim 1, wherein the alignment film includes a first layer having the surface and a second layer, and wherein the ratio of oxygen atoms at a position with the highest oxygen concentration in the first layer is 25% or higher than at a position with the lowest oxygen concentration in the first layer of the alignment film.
 3. The liquid crystal display device according to claim 1, wherein the alignment film includes a first layer having the surface and a second layer, and wherein the thickness of the first layer is smaller than the entire thickness of the alignment film by 50% or less.
 4. The liquid crystal display device according to claim 1, wherein the surface includes polyimide of (Chemical formula 1), and

in which the inside of the parentheses [ ] represents a chemical structure of a repeating unit, the index n represents a number of a repeating unit, N represents nitrogen atoms, O represents oxygen atoms, A represents a quadrivalent organic group including a cyclobutane ring, D represents a divalent organic group, and n is integer and one or greater than one.
 5. The liquid crystal display device according to claim 1, wherein a root-mean-square of a size of irregularities on the surface of the alignment film is 1 nm or less.
 6. The liquid crystal display device according to claim 1, wherein the alignment film includes a first layer having the surface and a second layer, and wherein the first layer is a photo-alignment upper layer capable of photo-alignment and the second layer is smaller in resistivity than the photo-alignment upper layer.
 7. The liquid crystal display device according to claim 1, wherein the liquid crystal display device is an In-plane switching (IPS) liquid crystal display device.
 8. The liquid crystal display device according to claim 1, wherein a contact angle of water on the surface of the alignment film is 38 degrees or more.
 9. The liquid crystal display device according to claim 4, wherein the A of the (Chemical formula 1) represents formula 2 and the D of the (Chemical formula 1) represents formula 4 or 5,


10. The liquid crystal display device according to claim 4, wherein the A of the (Chemical formula 1) represents formula 2 and the D of the (Chemical formula 1) represents formula 4,


11. The liquid crystal display device according to claim 4, wherein the second layer includes polyimide of (Chemical formula 6), and

in which the inside of the parentheses [ ] represents a chemical structure of a repeating unit, the index n represents a number of a repeating unit, N represents nitrogen atoms, O represents oxygen atoms, A represents formula 7, D represents a divalent organic group, and n is integer and one or greater than one, 